Conventionally, the development of a pharmaceutical agent has been vigorously carried out for incorporating an oil-soluble drug as an active ingredient into a base material. In particular, since the fact that corneocyte lipids existing on the surface of the skin, such as ceramide, are deeply associated with the barrier function of the stratum corneum has been revealed, the development of a pharmaceutical agent, into which such corneocyte lipids are mixed, has been attempted. However, such ceramides have high crystallinity, and thus, when they have been incorporated into a cosmetic product, the incorporated amounts have been limited from the viewpoint of stability. As a result, the effect of incorporating such ceramides has not been sufficiently obtained. Hence, it has been desired to develop a cosmetic product, which maintains high stability and which does not cause problems regarding the precipitation of crystals and the like even if ceramides are incorporated therein in amounts allowing the effect of the ceramides to be sufficiently expressed.
In order to achieve the object, there have been studied, for example, a method for finely and stably incorporating a lipid by using a nonionic surfactant and an ionic surfactant in combination (see Patent Document 1, for example), a method for forming liquid crystal using a lipid, a surfactant, and an oil (see Patent Document 2, for example), a method using a complex obtained by precipitating a lipid and a surfactant from an organic solvent (see Patent Document 3, for example), a method using a liposome (a vesicle consisting of a phospholipid bimolecular membrane), and the like.
However, in general, ceramides such as sphinganine have poor solubility in an oil. Accordingly, even in the aforementioned methods, relatively large amounts of surfactants and solvents need to be incorporated, and thus the produced cosmetic products have been problematic in terms of safety. In particular, when such ceramides are incorporated into a cosmetic product, which hates the incorporating of an organic solvent, a technique of completely removing a large amount of organic solvent used to obtain a complex consisting of a lipid and a surfactant has been necessary, and complicated operations should have been conducted in the production process of the cosmetic product. Moreover, as for the method using a liposome, since a phospholipid is generally an unstable substance, it has been difficult to sufficiently ensure high-temperature stability and long-term stability, when the liposome has been incorporated into a cosmetic product.
Patent literature 1: Japanese Unexamined Patent Publication H4-193814
Patent literature 2: Japanese Unexamined Patent Publication H6-345633
Patent literature 3: Japanese Unexamined Patent Publication H11-199462